The single stage fly-back PFC (power factor correction) converter is usually used in the existing low-cost LED driver to convert the mains voltage to a DC current that an LED light-emitting module needs. However, the single stage fly-back PFC converter cannot be adapted to the change of the 50/60 Hz mains voltage in order to keep a proper power factor and THD (total harmonic distortion) performance. This makes the power conversion from the mains voltage to the LED module inconstant over time, causing the output current supplied to the LED module to include quite amount of ripple current.
The amplitude of the output ripple current depends on the output capacitance of the single stage fly-back PFC converter used and effective series resistance of the LED module. The frequency of the output ripple current is generally 100/120 Hz, double of the input voltage of the fly-back PFC converter. Besides, a phase lead may be present between the output ripple current and the mains voltage as the load characteristics of the LED module is resistive and capacitive. These problems often will lead to high ripple ratio (generally more than 35%), and thus cause low reliability and quality of the LED module, and flickering concerns in camera or video application as well.
The above-mentioned problems are usually solved in the prior art using a single stage fly-back converter or a double stage design with a considerable output capacitance. For the first type of prior art, if the effective series resistance of the LED module is 6 Ohm and the output capacitance of the fly-back is 500 μF, the resulting ripple ratio is 47%; when the output capacitance is doubled to 1000 μF, the resulting ripple ratio is 26%; and when the output capacitance is tripled to 1500 μF, the resulting ripple ratio is 17%. However, this solution becomes quite unfavorable when the effective series resistance of the LED module is less and less. Besides, more capacitance requires more receiving space of PCB and bigger housing, moreover, the increased capacitance may lead to the problems such as power up delay and short circuiting current on the circuit.
In a second solution of the prior art, the combination of either a boost PFC module plus a fly-back module or a fly-back PFC module plus a buck module is often used, in which the first stage will provide a good power factor, THD, and a stable DC bus voltage, while the second stage will convert the bus voltage to a DC current and supply it to the LED module. Although no ripple current is at the LED driver output in this solution, compared with the low-cost single stage design, the double stage design has a relatively high cost and does not have a compact structure. Furthermore, the double stages design has an efficiency lower than that of the single stage design in the actual application, and is also subjected to EMI problem.